Huh, this comes from old devices like iPhone 4 or iPad 2...

The report of our games says that this sometimes happen on
iPad2, iPad2, iPad3, iPad4, iPadMini, iPhone4S, iPhone5

CC @wasedaigo

https://discussions.apple.com/thread/3908778
https://www.khronos.org/opengl/wiki/Framebuffer_Object_Extension_Examples

//Does the GPU support current FBO configuration?
//Before checking the configuration, you should call these 2 according to the spec.
//At the very least, you need to call glDrawBuffer(GL_NONE)
glDrawBuffer(GL_NONE);
glReadBuffer(GL_NONE);

EDIT: glDrawBuffer(s) doesn't exist on OpenGL ES 2 🤔

https://stackoverflow.com/questions/15935651/certain-framebuffer-sizes-fail-on-ios-devices-gl-framebuffer-unsupported

The framebuffer size width or height must be > 8 on some iOS devices.

And framebuffer sizes of our game are:

16 16
1024 1024
16 16
1 20
2048 2048
2304 54
480 720
16 16

So, that's the cause?

Fixed!